The invention relates to magnetic resonance (MR) imaging, and more particularly relates to MR angiography. In its most immediate sense, the invention relates to MR angiography of the legs or any other regions wherein bloodflow is regurgitated during the patient's cardiac cycle.
In conventional MR angiography of the arteries, a radiologist seeks to image a slice of interest in such a manner that arterial bloodflow contributes to the image while venous bloodflow does not. This has been done by saturating a slab of the patient immediately adjacent, and arterially downstream of, the slice of interest. The slab so established is known as a "saturation slab".
Saturated blood does not contribute to an MR image. Because the saturation slab is upstream of the slice of interest in the direction of venous bloodflow, venous blood passes through the saturation slab before it enters the slice of interest. Thus, venous bloodflow is 0 saturated by the time it enters the slice of interest and does not contribute to the MR image of the slice of interest. Because the saturation slab is downstream of the slice of interest in the direction of arterial bloodflow, arterial blood in the slice of interest has not previously passed through the saturation slab and therefore is not saturated. As a result, arterial blood contributes to the MR image of the slice of interest. In this way, the MR image of the slice of interest shows arterial bloodflow without venous bloodflow.
However, this technique has a disadvantage when it is used in regions where arterial bloodflow is regurgitated (reversed in direction). For example, let it be assumed that this known technique is used to conduct an MR angiography study of the arteries in a patient's legs, where arterial bloodflow is regurgitated during the patient's cardiac cycle. In this instance, arterial blood will flow into the saturation slab after leaving the slice of interest and will become saturated. During regurgitation, this saturated arterial blood will be withdrawn back into the slice of interest, where it will make no contribution to the MR image. As a result, the MR image of the slice of interest will be degraded.
It would be advantageous to provide method and apparatus which would not produce degraded MR angiographic images when used in locations where arterial blood is regurgitated.
It is, accordingly, one object of the invention to provide method and apparatus for producing an MR angiographic image of a slice of interest in a living patient in such a manner that the MR image is not degraded by regurgitated bloodflow.
Another object is, in general, to improve on known MR methods and apparatus of this general type.
In accordance with the invention, the saturation slab is either moved or eliminated during a single MR sequence. Advantageously, and in accordance with the preferred embodiments, the motion is carried out as a function of the cardiac cycle, or the saturation slab is eliminated, before the regurgitated bloodflow takes place.
By so moving the saturation slab, or by so eliminating it, the regurgitated blood is not saturated and therefore contributes to the image of the slice of interest when such regurgitated blood re-enters the slice of interest.